The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the reduction of motion artifacts in NMR images produced using Carr-Purcell-Meiboom-Gill (CPMG) pulse sequences.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant .gamma. of the nucleus). Nuclei which exhibit this phenomena are referred to herein as "spins".
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M.sub.z is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1 which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation signal B.sub.1 is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance (NMR) phenomena is exploited.
When utilizing NMR to produce images, a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region which is to be imaged (region of interest) is scanned by a sequence of NMR measurement cycles which vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. To perform such a scan, it is, of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (G.sub.x, G.sub.y, and G.sub.z) which have the same direction as the polarizing field B.sub.0, but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified.
Most NMR scans currently used to produce medical images require many minutes to acquire the necessary data. The reduction of this scan time is an important consideration, since reduced scan time increases patient throughput, improves patient comfort, and improves image quality by reducing motion artifacts. There is a class of pulse sequences in which a complete scan can be conducted in seconds rather than minutes.
The concept of acquiring NMR image data in a short time period has been known since 1977 when the echo-planar pulse sequence was proposed by Peter Mansfield (J. Phys. C.10: L55-L58, 1977). In contrast to standard pulse sequences, the echo-planar pulse sequence produces a set of NMR signals for each RF excitation pulse. These NMR signals can be separately phase encoded so that an entire scan of 64 views can be acquired in a single pulse sequence of 20 to 100 milliseconds in duration. The advantages of echo-planar imaging ("EPI") are well-known, and variations on this pulse sequence are disclosed in U.S. Pat. No. 4,678,996; 4,733,188; 4,716,369; 4,355,282; 4,588,948 and 4,752,735.
One variant of the echo planar imaging method is the Rapid Acquisition Relaxation Enhanced (RARE) sequence which is described by J. Hennig et al in an article in Magnetic Resonance In Medicine 3,823-833 (1986) entitled "RARE Imaging: A Fast Imaging Method For Clinical MR." The essential difference between the RARE sequence and the EPI sequence lies in the manner in which echo signals are produced. The RARE sequence utilizes RF refocused echoes generated from a Carr-Purcell-Meiboom-Gill (CPMG) sequence, while EPI methods employ gradient recalled echoes.
Movement of the spins being imaged can produce artifacts in the reconstructed image. Such movement may occur, for example, by patient respiration during the scan or it may occur during each pulse sequence as a result of CSF and blood flow. Flowing spins can introduce phase errors into the acquired NMR data set which result in the production of ghosts or in blurring of the reconstructed image. Such errors are particularly troublesome with CPMG sequences and the image artifacts are, at times, worse than with images reconstructed from conventionally acquired NMR pulse sequences.
One method for reducing motion artifacts in conventionally acquired NMR images is referred to in the art as "gradient moment nulling". This method requires the addition of gradient pulses to the pulse sequence which cancel, or null, the effect on the NMR signal phase caused by spins moving in the gradients employed for position encoding. Such a solution is disclosed, for example, in U.S. Pat. No. 4,731,583 entitled "Method For Reduction of NMR Image Artifacts Due To Flowing Nuclei By Gradient Moment Nulling". Other references which describe gradient moment nulling in NMR pulse sequences include:
P. M. Pattany, J. J. Phillips, L. C. Chiu, J. D. Lipcamon, J. L. Duerk, J. M. McNally, S. N. Mohapatra, J. Comput. Assit. Tomogr. 11: 367-377 (1987). PA0 E. M. Haake, G. W. Lenz, Am. J. Roentgenology 148: 1251-1258 (1987). PA0 D. G. N'ishimura, J. I. Jackson, J. M. Pauly, Mag. Res. Med. 22: 481-492 (1991). PA0 J. L. Duerk, O. P. Simonetti, J. Mag. Res. Imag. 1: 643-650 (1991).
When gradient moment nulling is applied to CPMG pulse sequences the result is unsatisfactory. Some reduction in flow artifacts usually results, but the amount of improvement varies considerably from scan-to-scan and the improvement is seldom as good as when gradient moment nulling is applied to conventional NMR pulse sequences.